This invention relates generally to gas turbine engines and more particularly to and adaptive cycle gas turbine engine including an aft fan.
A gas turbine engine includes a turbomachinery core that is operable in a known manner to generate a primary flow of propulsive gas. A typical turbofan engine adds a low pressure spool with a turbine driven by the core exhaust gases that in turn drives a fan through a shaft to generate a bypass flow of propulsive gas. A turbofan engine may be characterized as “low bypass” or “high bypass” based on the ratio of bypass flow to core flow.
Low-bypass turbofan engines are commonly used in military aircraft. Increasingly, emphasis is being applied to low-observable and integrated powerplant technology. These considerations drive the use of embedded installations in which an engine is “buried” deep within an airframe (as opposed to being mounted in a pod or nacelle) and may have indirect and long inlet and exhaust ducts.
Advanced military aircraft concepts will require improved performance moderate- to high-bypass turbine engines in embedded installations. Performance requirements are demanding engine cycles with increasingly higher overall pressure ratios, but with low pressure ratio fans. Prior art engine architectures require a large number of stages in the low pressure spool in order to achieve a high overall pressure ratio while maintaining relatively low rotational speeds required by the fan. Fans in such engines are large diameter and therefore create difficulties in the sharp turns required in the embedded inlet due to the short distance from the front of the vehicle to the fan face. Other vehicle requirements such as large air offtakes for high lift devices, large mechanical and/or electrical power extraction for aircraft mission systems and high thermal loads are difficult to meet with a fixed cycle turbine engine.
Accordingly, there is a need for an adaptive cycle, high performance turbine engine suitable for use in embedded installations.